1. Field of the Invention
The present invention relates to a steering system for simultaneously steering the front and rear wheels of an automotive vehicle.
2. Description of the Relevant Art
There are known steering systems for steering the rear wheels of an automotive vehicle simultaneously with the front wheels thereof (also referred to as a "4WS system"), as disclosed in Japanese Patent Publication No. 60-44185, for example. When the automotive vehicle is steered while it is traveling at a high speed, the rear wheels are steered in the same direction as the steering direction of the front wheels, and hence the steering response is increased. When the vehicle is steered at a low vehicle speed, the rear and front wheels are steered in opposite directions, thus reducing the radius of turning movement of the vehicle, thereby to improve the vehicle maneuvering ability.
The 4WS systems with the steering angle ratio between the front and rear wheels being variable as a function of the vehicle speed can roughly be divided into two categories. In one category, front and rear wheel steering mechanisms are driven by independent drive sources such as electric motors or hydraulic actuators and the rear wheels are steered by the rear wheel steering mechanism based on a front wheel steering angle and vehicle speed information (see Japanese Laid-Open Patent Publications Nos. 59-81264 and 59-81274, for example). In the other category, front and rear wheel steering mechanisms are mechanically interconnected and an additional drive device for adjusting the rear wheel steering angle dependent on the vehicle speed is associated with the rear wheel steering mechanism (see U.S. Pat. No. 4,610,455 and Japanese Laid-Open Patent Publications Nos. 59-26363 and 59-26364, for example).
The 4WS system in the former group requires the independent drive source for steering the rear wheels and hence is of an increased weight and takes up an increased space in the automotive vehicle. In addition, this type of 4WS system fails to provide a good response since the steering angles of the front and rear wheels and the vehicle speed must be detected from time to time and the steering angle ratio and the vehicle speed must be compared.
According to the 4WS system of the latter design, inasmuch as the front and rear wheel sterring mechanisms are mechanically coupled to each other, an increase in the steering force that is required to steer the rear wheels together with the front wheels can be produced simply by increasing the capability of the power assisting device in the front wheel steering mechanism. Therefore, the 4WS system of this type is lighter and occupies a smaller space than the 4WS system of the former type does. Another advantage is that the steering response is quick because the rear steering angle responsive to the front steering angle is adjusted dependent on the vehicle speed. However, one problem of the conventional 4WS systems in the latter category is the requirement of a relatively large driving force for the rear wheel driving mechanism since the rear wheel steering angle is adjusted by vertically moving the output member of the rear wheel driving mechanism.